[M2] STI/CLI wrappers + interrupt smoke test

[mitsuakki]

Add enable_interrupts()/disable_interrupts() wrappers around sti/cli.

Then the integration test: flip interrupts on in QEMU and confirm three things:.

• a forced exception still hits the panic screen
• the timer tick counter increments without hanging
• keypresses produce scancodes through IRQ1.

Expected QEMU output should get written down somewhere (issue or PR) to catch regressions later.

[mitsuakki]

test n°3 will need a whole keyboard driver implementation